1. Field of the Invention
The invention relates to brazing sheet having a core sheet of an aluminium alloy core material and a brazing layer of an aluminium alloy having silicon as the main alloying element on at least one side of the core sheet. The invention further relates to a method of making such a brazing sheet and to a process for manufacturing an assembly from the brazing sheet, as well as to an assembly thus manufactured.
2. Description of the Prior Art
Brazing sheet of this type is known and is used for automotive radiators, amongst other products. Two brazing methods, known as Nocolok and vacuum brazing, are conventional and need not be described here. Such brazing takes place at a temperature of about 600.degree. C. as determined by the aluminium silicon alloy of the brazing layer.
The brazing sheet disclosed by EP-A-326 337 consists of an AA 3005 type aluminium alloy core material and a brazing layer of an aluminium alloy having silicon as the main alloying element. This brazing sheet has a good corrosion resistance after brazing because a silicon-rich band of precipitates in the core material is formed near to the interface between the brazing layer and the core material by diffusion of silicon out of the brazing layer into the core material during brazing. The conditions for obtaining the silicon-rich band in the core material specified in EP-A-326 337 are limitations of the silicon and iron content in the core material of Si &lt;0.15% and Fe &lt;0.40% and further a manufacturing process of the brazing sheet in which the brazing sheet is not homogenized before hot rolling and is not interannealed after hot rolling. The copper content of the core material is about 0.3% (herein all composition amounts are % by weight).
From WO94/22633 there is known an improvement of the brazing sheet of EP-A-326 337 having improved yield strength whilst the corrosion resistance is maintained. According to this publication the post-brazed yield strength of the brazing sheet of EP-A-326 337 is 55 MPa whilst the brazing sheet of WO94/22633 has a post-brazed yield strength in the range of 54-85 MPa. This is obtained at a copper content of 0.6% whilst the silicon content is kept &lt;0.15% to maintain the corrosion resistance properties.
Today the weight of cars should be reduced to reduce fuel consumption and air pollution. In connection therewith the weight and dimensions of automotive radiators and their cooling fluid content should be reduced by means of thinner brazing sheet having improved strength properties.
There are known aluminium alloys having better strength properties than those discussed above, but the present applicants believe that they do not meet the requirement of corrosion resistance when used as core material for brazing sheet.
Further prior art documents to be discussed in relation to the present invention are as follows.
JP-A-4-263033 specifies a brazing material having an aluminium core of the composition
Mn 0.3-1.5 PA1 Cu 0.2-0.9 PA1 Mg 0.2-0.5 PA1 Si 0.2-1.0 PA1 Fe 0.1-0.7 PA1 Ti 0.1-0.3 PA1 optionally Zr 0.05-0.2 PA1 optionally Cr 0.05-0.2 PA1 balance Al. PA1 Si 0.05-0.8 PA1 Fe 0.05-0.6 PA1 Cu 0.1-1.0 PA1 Mn 0.6-1.6 PA1 Mg 0.05-0.5 PA1 optionally Cr, Zr, Ti each .ltoreq.0.3 PA1 balance Al. PA1 Cu 0.1-0.6 PA1 Mg 0.2-0.6 PA1 Mn 0.3-1.5 PA1 Si 0.3-0.6 PA1 optionally Zr, Cr, Ti PA1 balance Al. PA1 Ti 0.11-0.30 PA1 Mn 0.3-1.5 PA1 Cu 0.4-0.6 PA1 Fe .ltoreq.0.7 PA1 Si .ltoreq.0.8 PA1 Mg .ltoreq.1.5 PA1 balance Al. PA1 Fe 0.1-0.7 (in preferred composition 0.2-0.5) PA1 Mn 1-1.5 PA1 Si 0.2-0.5 (in preferred composition 0.2-0.4) PA1 Cu 0.2-0.5 (in preferred composition 0.25-0.5) PA1 Mg .ltoreq.0.5 PA1 Ni 0.4-1.0 PA1 Cr .ltoreq.0.5 PA1 Zr .ltoreq.0.4 PA1 Ti 0.01-0.1 PA1 V .ltoreq.0.4 PA1 others total 0.15 PA1 balance Al. PA1 Mn 0.3-2.0 PA1 Cu 0.25-0.8 PA1 Si 0.2-1.0 PA1 Mg .ltoreq.0.5 PA1 Ti .ltoreq.0.35 PA1 balance Al. PA1 Mn 0.7-1.5 PA1 Cu 0.2-2.0 PA1 Mg 0.1-0.6 PA1 Si &gt;0.15 PA1 Fe up to 0.8 PA1 Ti optional, up to 0.15 PA1 Cr optional, up to 0.35 PA1 Zr and/or V optional, up to 0.25 in total PA1 balance Al and unavoidable impurities, PA1 with the proviso that (Cu+Mg)&gt;0.7, PA1 (i) casting the aluminium alloy core material PA1 (ii) applying the brazing layer or layers to a sheet of the aluminium alloy core material PA1 (iii) hot rolling the aluminium alloy core material sheet and brazing layer or layers PA1 (iv) cold rolling the hot rolled product from step (iii) PA1 (i) forming parts of which at least one is made from the brazing sheet, PA1 (ii) assembling the parts into the assembly, and PA1 (iii) brazing the assembly, PA1 (iv) the part or parts made from brazing sheet are at least partly made from brazing sheet in accordance with this invention described above, and PA1 (v) the assembly is subjected to ageing after the brazing of step (iii).
On one side this core has a conventional Al--Si brazing or filler layer, and on the other side it has a sacrificial anode layer of Al--Zn--Mg, whose purpose is to reduce corrosion of the core layer when in contact with water. The inclusion of Mg in the sacrificial anode layer is to permit Mg to diffuse into the core during brazing. Mg content in the core is generally at a low level, to prevent its diffusion into the brazing layer. The brazing material is made with an intermediate annealing step between hot rolling and cold rolling.
JP-A-4-202735 describes a similar brazing sheet having a sacrificial anode layer. The core material has the composition
The document discusses formation of intermetallic compounds in the core material to improve strength and corrosion resistance. Homogenization and annealing were employed in the production of the sheet, presumably to ensure formation of the intermetallic compounds. It is stated that up to 1.5% Mg may be added to the sacrificial anode layer, to prevent diffusion of Mg from the core into this layer.
JP-A-2-129333 also describes a brazing sheet having a sacrificial anode layer of Al--Zn--Mg on the aluminium alloy core, but optionally the core may instead have a brazing layer on both sides. The core alloy composition is
In relation to the present invention, it is commented that, in the two examples in which the sacrificial anode layer is absent, (Cu+Mg) in the core is 0.48 and 0.40 respectively. Homogenization and annealing steps are both included in the manufacture of the sheet.
U.S. Pat. No. 4,649,087 describes a core alloy for a brazing material, having the composition
The concept described is the inclusion of Ti at higher levels, in order to improve corrosion resistance. In the examples, the core alloy was homogenized prior to hot rolling and intermediate annealing was performed between hot and cold rolling.
WO82/01014 describes a core alloy for brazing material in which Ni is present. The composition is
In the preferred composition, (Cu+Mg) is 0.4-0.8. In one example (Cu+Mg) is 0.6, in another 0.65. In two others Mg &lt;0.05. The document proposes a high temperature post-brazing treatment at 450.degree.-550.degree. C. The method of manufacture includes homogenization before hot rolling.
EP-A-241125 similarly describes a post-brazing heat treatment. In the examples, the Cu and Si levels in the core material are low.
JP-A-4-198448 is another document disclosing a brazing material having a sacrificial anode layer containing Mg on an aluminium alloy core material. The core material has the composition
The sacrificial anode layer contains Mg 1.2-2.5 and Si 0.2-0.8. It is stated that Mg in the sacrificial anode layer diffuses into the core during brazing.